Activates an agent for a session identified by fid.
This is the entry point for UI to change agents. UI calls this method, which routes to session.activate_agent() to handle the agent change, including copying chat/messages from old AgentHandler to new AgentHandler.
| fid |
The session identifier (file ID) |
| agent_name |
The name of the agent to activate |
| Error |
if session not found or agent activation fails |